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Abstract

In this article, the electroluminescence (EL) spectra of zinc oxide (ZnO) nanotubes/p-GaN
light emitting diodes (LEDs) annealed in different ambients (argon, air, oxygen, and
nitrogen) have been investigated. The ZnO nanotubes by aqueous chemical growth (ACG)
technique on p-GaN substrates were obtained. The as-grown ZnO nanotubes were annealed
in different ambients at 600°C for 30 min. The EL investigations showed that air,
oxygen, and nitrogen annealing ambients have strongly affected the deep level emission
bands in ZnO. It was concluded from the EL investigation that more than one deep level
defect is involved in the red emission appearing between 620 and 750 nm and that the
red emission in ZnO can be attributed to oxygen interstitials (Oi) appearing in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV), and to oxygen
vacancies (Vo) appearing in the range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The annealing
ambients, especially the nitrogen ambient, were also found to greatly influence the
color-rendering properties and increase the CRI of the as - grown LEDs from 87 to
96.

Introduction

Zinc oxide (ZnO) is a direct wide band gap (3.37 eV) semiconductor. In recent years,
it has attracted the attention of the research community for a variety of practical
applications due to its excellent properties combined with the facility of growing
it in the nanostructure form.

At present, ZnO is considered to be a very attractive material because it combines
semiconducting and piezoelectric properties and in addition it is transparent, biocompatible,
and bio-safe. These unique properties of ZnO makes it as a promising candidate for
the next generation of visible and ultra-violet (UV) light-emitting diodes (LEDs)
and lasing devices. The visible emission results because ZnO possesses deep level
emission (DLE) bands and emit all the colors in the visible region with good color-rendering
properties [1-8]. It is important to understand the origin of the emissions related to deep level
defects in ZnO for the development of optoelectronic devices with high efficiency.

A number of studies on the optical properties of ZnO nanostructures have suggested
that, within the DLE, the green (approximately 500 nm) and red (approximately 600
nm) emissions have originated from oxygen vacancies (Vo) and zinc interstitial (Zni) [9-14]. Other authors have reported that the green emission can be attributed to both oxygen
and zinc vacancies [15,16]. The violet-blue and blue emissions were attributed to zinc interstitial (Zni) and Zinc vacancies (Vzn), respectively, in the DLE [17-19]. The yellow emission in hydrothermally grown nanorods was attributed to the presence
of OH groups on the surface [9]. The formation energy and energy levels of different defects within the DLE have
been experimentally studied and calculated by other authors [9,20]. However, the origins of different defect emissions are still not fully understood,
and the hypotheses that have been proposed to explain the different defect emissions
(violet, blue, green, yellow, orange-red, and red) have been controversial [9,10,21,22]. Therefore, still a considerable interest is being shown in investigating the defect
emissions in ZnO in general and, ZnO nanostructures in particular, because of their
great potential for optical applications.

The ZnO nanotubes are the best candidates for white LEDs among all of the known oxide
semiconductors, and they can be easily grown via chemical and other physical vapor-phase
approaches as well [6]. The small footprint and the large surface area-to-volume ratio make the ZnO nanotubes
a better candidate for heterojunction white LEDs as compared to thin films. The lattice
mismatch can be compensated in view of the favorable stress/strain values observed
for ZnO nanotubes as compared to thin films. A notable advantage of nanotube-based
LEDs is that each nanotube can act as a wave guide, minimizing the side scattering
of light, thus enhancing light emission and extraction efficiency [23]. The GaN has close lattice mismatch with ZnO, and the close lattice match is the
main factor that can influence the optical and electrical properties of heterojunctions.
Only a few studies focusing on n-ZnO nanotubes, on p-GaN, and on white light emitting
diodes (LEDs) are available in the literature [24-26].

Many researchers have investigated the DLEs in ZnO. The optical properties of chemically
synthesized ZnO nanorods, post-growth annealed in temperatures ranging from 200 to
800°C, have been studied using photoluminescence measurements. In our investigation,
the as-grown nanotubes were annealed at 600°C as this temperature was found to be
very effective in modifying the DLEs [9,10,21,27,28]. Previously, the authors have investigated the effect of post-growth annealing treatment
on the electroluminescence (EL) of n-ZnO nanorods/p-GaN LEDs. The annealing ambients
have the same effect on EL of LEDs, but ZnO nanotube-based LEDs were found to have
approximately twice the EL intensity as compared to that of ZnO nanorod-based LEDs
[29].

ZnO nanostructures grown by low temperature (<100°C) growth techniques such as aqueous
chemical growth (ACG) have low crystal quality with lattice and surface defects. The
post-growth annealing is an effective tool to enhance and control the crystallinity
and optical properties of ZnO nanostructures [21]. In this article, the EL spectra of LEDs fabricated using the as-grown as well as
the ZnO nanotubes annealed in argon, air, oxygen, and nitrogen ambients have been
investigated. The results showed that oxygen and nitrogen ambients are very effective
on modifying the deep level defects, and that the red emission in ZnO was attributed
to the superposition of emissions related to oxygen interstitial and oxygen vacancies
in ZnO. The post-growth annealing ambient also strongly influences the color-rendering
properties of ZnO nanotubes. We have commercially purchased magnesium-doped p-type
GaN with film thickness of 4 μm on sphire substrates from TDI Inc. USA. It has hole
concentration of approximately 4 × 1017 cm-3.

To obtain the ZnO nanotubes, first, the ZnO nanorods were grown on the p-GaN substrates
using the low temperature ACG method, and then these nanorods were chemically etched
to get nanotubes. There are many chemical growth methods employed for growing ZnO
nanorods. The most common method is the one described by Vayssieres et al. [30]. By using this method, the ZnO nanorods were grown on p-GaN substrate. To improve
the quality of the grown ZnO nanorods, the said method was combined with the substrate
preparation technique developed by Greene et al. [31]. The grown ZnO nanorods on the p-GaN substrates were etched by placing the samples
in 5-7.5 molar KCl (Potassium chloride) solution for 5-10 h at 95°C.

The samples were then annealed in argon, air, oxygen, and nitrogen ambients at 600°C
for 30 min. Pt/Ni/Au alloy was used to form ohmic contact with the p-GaN substrate.
The thicknesses of the Pt, Ni, and the Au layers were 20, 30, and 80 nm, respectively.
The samples were then annealed at 350°C for 1 min in flowing argon atmosphere. This
alloy gives a minimum specific contact resistance of 5.1 × 10-4 Ω cm-2 [32]. An insulating photo-resist layer was then spun coated on the ZnO NTs to fill the
gaps between the nanotubes with a view to isolate electrical contacts on the ZnO NTs
to prevent them from reaching the p-type substrate, thereby helping to prevent the
carrier cross talk among the nanotubes. To form the top contacts, the tip of the ZnO
NTs were exposed using plasma ion-etching technique after the deposition of the insulating
photo-resist layer. Non-alloyed Pt/Al metal system was used to form the ohmic contacts
to the ZnO NTs. The thicknesses of the Pt and the Al layers were 50 and 60 nm, respectively.
This contact gives a minimum specific contact resistance of 1.2 × 10-5 Ω cm-2 [28]. The diameter of the top contact was about 0.58 mm.

Results and discussions

Figure 1a,b shows the images of the top of the ZnO nanotubes before and after annealing, respectively.
The figure shows clearly the morphology and size distribution of the as-grown ZnO
nanotubes. Hexagonal, well-aligned, vertical ZnO nanotubes were obtained on the p-GaN
substrate. The ZnO NTs grown had a uniaxial orientation of 〈0001〉 with an epitaxial
orientation with respect to the p-GaN substrate, forming n-ZnO-(NTs)/p-GaN p-n heterojunctions.
From the SEM images, the mean inner and outer diameters of the as-grown ZnO nanotubes
in this study were found to be approximately 360 and 400 nm, respectively. Figure
1c shows the current-voltage, I-V, curves of the n-ZnO NTs/p-GaN LEDs developed in this study. All the LEDs have the
same I-V curves. The I-V curves clearly show a rectifying behavior of the LED as expected with a turn on threshold
voltage of about 4 V. This indicates clearly that both metal/GaN and metal/n-ZnO interfaces
have formed good ohmic contacts. Figure 1d shows the schematic illustration of the fabricated LEDs.

Figure 1.SEM image of ZnO nanotubes on p-GaN substrate. (a) before annealing, (b) after annealing, (c) typical I-V characteristics for the fabricated LEDs, and (d) The schematic illustration of the fabricated LEDs.

Figure 2 shows the EL spectra of the as-grown and annealed LEDs. All the EL measurements were
taken under forward bias of 25 V. The EL spectra consist of violet, violet-blue, orange,
orange-red, and red peaks. The violet and violet-blue peaks are centered approximately
at 400 nm (3.1 eV) and 452 nm (2.74 eV), respectively. The broad green, orange, orange-red,
and red peaks are centered approximately at 536 nm (2.31 eV), 597 nm (2.07 eV), 618
nm (2.00 eV), and 705 nm (1.75 eV), respectively. The EL emission in the ultraviolet
(UV) region was not detected here since the authors were interested only in the visible
emissions; therefore, the lower EL detector limit was set to 400 nm.

Figure 2.Electroluminescence spectra of the LEDs at an injection current of 3 mA for the as
grown and annealed ZnO NTs in different ambients under forward bias of 25 V and it
shows the shift in emission peak after annealing in different ambient.

The EL intensity of the samples annealed in argon is low compared to the as-grown
and all other samples annealed in different ambients. The ZnO nanotubes having low
growth temperature (<100°C) possess many intrinsic defects, such as oxygen vacancy
(Vo), zinc vacancy (Vzn), interstitial zinc (Zni), interstitial oxygen (Oi), etc., and these defects are responsible for the DLEs. These defects are reduced
after annealing at high temperature (600°C). Such activation or passivation of intrinsic
defects would greatly enhance the crystal's deep level defect structure leading to
the modification of luminescence spectra efficiency of the LEDs [16]. This argument is also confirmed by the EL spectra obtained for ZnO nanotubes annealed
in argon (see Figure 2). The EL intensities of the violet (400 nm) and violet-blue (452 nm) of all the annealed
samples are decreased as compared with the as-grown samples. In the literature, it
was reported that the violet emission from undoped ZnO nanorods is related to Zinc
interstitial (Zni) [22]. The violet peak is centered at 3.1 eV (400 nm), and this agrees well with the transition
energy from Zni level to the valence band in ZnO (approximately 3.1 eV). The violet-blue peak was
centered at 2.74 eV (452 nm) for all the EL measurements in different ambients. It
is attributed to recombination between the Zni energy level to the VZn energy level, and approximately is in agreement with the transition energy from Zni energy level to VZn energy level (approximately 2.84 eV). There is a difference of 0.11 eV. This difference
maybe is due to the effect of GaN substrate, as GaN also emits blue light. There are
no shifts in violet and violet-blue peaks after annealing in different ambients. The
violet and violet-blue emissions decreased after annealing the as-grown ZnO nanotubes
in different ambients. The violet and violet-blue are the high energy emissions in
the visible region, and the annealing affects the deep level defects that are responsible
for low energy emissions from the green-to-red region in the visible spectra (see
in Figure 2). It increases the transition recombination rate for the deep level defects that
are responsible for the green-to-red emissions. Therefore, the EL intensities of the
DLEs (the green to red) are increased, while those of the violet and violet-blue emissions
are decreased after annealing in different ambients. Only for the case of the argon
ambient, all the defects are modified, and owing to this, the El intensities of all
the emissions decreased after annealing.

The broad green peak, centered at 536 nm (2.31 eV) in the EL spectra of the as-grown
ZnO nanotube-based LEDs and LEDs based on annealed ZnO nanotubes in argon ambient,
is attributed to oxygen vacancy (Vo). It is believed that this phenomenon is due to band transition from zinc interstitial
(Zni) to oxygen vacancy (Vo) defect levels in ZnO [22]. This has been explained by the full potential linear muffin-tin orbital method,
which posits that the position of the Vo level is located approximately at 2.47 eV below the conduction band, and the position
of the Zni level is theoretically located at 0.22 eV below the conduction band. Therefore, it
is expected that the band transition from Zni to Vo level is approximately 2.25 eV [22]. This agrees well with the green peak that is centered approximately at 2.31 eV.

The orange-red peaks are centered at 597 nm (2.07 eV) and 618 nm (2.00 eV) for the
samples annealed in air and oxygen, respectively. These emissions are attributed to
oxygen interstitials Oi, and believed to be due to band transition from zinc interstitial (Zni) to oxygen interstitial (Oi) defect levels in ZnO [22]. The position of the Oi level is located approximately at 2.28 eV below the conduction band, and it is expected
that the band transition from Zni to Oi level is approximately 2.06 eV [22]. This agrees well with the orange-red peaks that are centered approximately at 2.00
and 2.07 eV.

The EL spectra of ZnO nanotubes annealed in oxygen and air ambients are nearly similar.
The EL intensity of the sample annealed in oxygen is higher compared to that of the
sample annealed in air. Its means that air and oxygen produce the same defects, but
the ratio of these defects is more in the case of oxygen. As the orange-red emission
is attributed to oxygen interstitials Oi [22], the annealing in oxygen ambient increases the amount of oxygen-related Oi defects; therefore, the orange-red emission dominates the EL spectra.

The red emission centered at 705 nm (1.75 eV) can be attributed to oxygen vacancies
(Vo). For the ZnO nanotubes annealed in nitrogen ambient, the following oxygen desorption
may occur;

The zinc vacancies are filled with zinc during the annealing of the ZnO nanotubes
in the nitrogen ambient. The majority of defects are oxygen vacancies (Vo) that are created by the evaporation of oxygen [21]. The red emission centering at 706 nm (1.75 eV) may be attributed to the transition
from oxygen vacancy (Vo) level to top of the valance band in ZnO. Using full-potential linear muffin-tin
orbital method, the calculated energy level of the Vo in ZnO is 1.62 eV below the conduction band [20]. Hence, the energy interval from the Vo energy level to the top of the valence band is approximately 1.75 eV. It agrees well
with that observed for the red emission centered at 1.75 eV.

By comparing the EL spectra of samples annealed in oxygen and nitrogen, it can be
concluded that the total red emission ranging from 620 nm (1.99 eV) to 750 nm (1.65
eV) is the combination of emissions related to Oi and Vo defects. The EL spectra of the samples annealed in oxygen show that after annealing,
the red emission is enhanced in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV)
when compared to the as-grown samples, and the EL spectra of the samples annealed
in nitrogen ambient show that, after annealing, the red emission is enhanced in the
range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The EL intensities of the green,
yellow, orange, and the red emission (from 620 to 690 nm) are decreased, but the EL
intensity of the red emission (from 690 to 750 nm) has increased significantly as
compared with the as-grown ZnO nanotubes. Therefore, it is clear that the red emissions
from 620 to 690 nm and from 690 to 750 nm have different origins. The red emission
in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi, and that in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) can be attributed
to Vo.

Figure 3a,b,c,d,e shows the CIE 1931 color space chromaticity diagram in the (x, y) coordinates system. The chromaticity coordinates are (0.3559, 0.3970), (0.3557,
3934), (0.4300, 0.4348), (0.4800, 0.4486), and (0.4602, 0.3963) with correlated color
temperatures (CCTs) of 4802, 4795, 3353, 2713, and 2583 K for the as-grown ZnO nanotubes,
annealed in argon, air, oxygen, and nitrogen, in the forward bias, respectively. The
chromaticity coordinates are very close to the Planckian locus which is the trace
of the chromaticity coordinates of a blackbody. The colors around the Planckian locus
can be regarded as white. It is clear that the fabricated LEDs are in fact the white
LEDs.

Figure 4 shows the schematic band diagram of the DLE emissions in ZnO, based on the full-potential
linear muffin-tin orbital method and the reported data.

Figure 4.Schematic band diagram of the DLE emissions in ZnO based on the full potential linear
muffin-tin orbital method and the reported data as described in references [9-20,22]. Also oxygen vacancies situated 1.65 eV below the conduction band are denoted to
be contributing to the red emission.

In summary, the origin of red emission in chemically obtained ZnO nanotubes has been
investigated by EL spectra. The as-grown samples were annealed in different ambient
(argon, air, oxygen, and nitrogen). It was observed that the post-growth annealing
in nitrogen and oxygen ambients strongly affected the green, yellow, orange, and red
emissions of ZnO nanotubes. The EL intensities of the green, the yellow, the orange,
and the red emissions were gradually increased after annealing in air, oxygen ambients,
and decrease in argon ambient. However, in nitrogen ambient, the EL emission of the
red peak in the range of 690--750 nm was increased, and in the range of 620-690 nm,
it was decreased as compared with the as-grown samples. It was found that more than
one deep level defect are involved in producing the red emission in ZnO.